The present invention relates generally to a circuit for monitoring the status of a fuse in an electric circuit and, in particular, to a circuit for monitoring the status of a plurality of fuses in a motor bridge circuit.
Electric power circuits are typically protected by a fuse connected between a power source and the electric circuit for limiting the maximum current which can be supplied to the circuit. In many instances, a circuit incorporates a plurality of fuses for separately protecting against the application of excess current to individual portions or components of a circuit.
In circuits which utilize more than a single fuse element, it is desirable to provide an indication to an operator in the event one of the fuses should blow. One such fuse indicating system is disclosed in U.S. Pat. No. 2,317,030 to Colvin. The Colvin fuse indicating system includes a separate current-limiting resistor and an associated indicator lamp connected in series across each fuse to be monitored. When one of the fuses blows, current passes through the current-limiting resistor, causing the indicator lamp to light. However, the Colvin system relies on the power source connected to the monitored fuse in order to light the associated indicator light.
A fuse loss indicating system which utilizes an auxiliary power supply to power an indicator light is disclosed in U.S. Pat. No. 3,512,073 to South. In the South patent, a transformer is coupled to actuate a switching means to light the indicator lamp in the event a monitored fuse blows. However, since only a simple indicator is provided, the operator has no indication as to which one of a plurality of fuses has blown.
A fuse testing circuit is disclosed in U.S. Pat. No. 3,407,293 to Tonnessen. The fuse testing circuit of the Tonnessen patent includes a plurality of relays for sequentially connecting the fuse testing circuit to the fuses to be tested. When one of the fuses has blown, a test of the blown fuse causes the output of a flip flop to be set to a logic "1" state. The logic "1" output of the flip flop triggers a warning circuit to energize a flashing lamp to alert the operator that a fuse has blown. While the Tonnessen circuit is capable of testing a plurality of fuses and providing an indication if one of the fuses should be blown, the circuit has at least two drawbacks. Since only one indicator lamp is utilized, the operator has no indication as to which one of a plurality of fuses has blown. Furthermore, the Tonnessen circuit is merely a fuse testing circuit, and therefore does not continuously monitor the condition of the fuses.
Other fuse loss indicating systems are disclosed in U.S. Pat. No. 3,045,167 to Colaiaco and U.S. Pat. No. 3,546,692 to Salzer.